The problem posed by this known technique is that the probability of detecting a transmission error on the channel, depending on the length of the CRC, is nevertheless ½32. Even by confirming the transmission through channel redundancy, the probability of non-detection remains 10−6 per hour in the case of a 100 Mbs Ethernet or of an AFDX (Avionics Full DupleX switched Ethernet) bus. This value is generally greater than what is required for transmission of flight critical data; that is to say, any piece of data in which the erroneous value can to trigger a scenario classed as catastrophic because it may lead to the loss of the aircraft and its passengers. Data such as altitude, velocity, variables involved in the flight control, fuel quantity, flow or temperature etc. are, for example, pieces of information which, in certain conditions depending on the aeroplane system using them, may cause inappropriate system responses capable of leading to such scenarios. The certification standards for aircraft used in commercial transport require demonstration that the probability of catastrophic scenarios occurring is less than 10−9 per flight hour. On-board communications networks causing several of these systems to communicate are led to carry information flows contributing to a significant quantity of such scenarios. For the modern aircraft in which such communications networks are used, it is acceptable to consider the network system a system in its own right and to allocate it intrinsic performances in terms of the integrity of data carried. It is generally agreed to set the maximum rate of undetected erroneous data at a value equal to 10−9 per flight hour to make such a communications system suitable for carrying critical information. Generally, for reasons of availability it is known to duplicate the communications network and to look for behaviour of each network consisting in disabling the frames or the erroneous data flows detected or even the network itself by relying on the second redundant network. It is therefore possible to think of disabling the communications channel responsible for exceeding an error threshold (which assumes detecting and accounting for the errors), but it is only possible to show that errors being reproduced in a systematic manner for particular data would be detected. In the following text, errors statically affecting a piece of data that is itself static such that the non-detection by the CRC, when it occurs, is reproduced indefinitely as long as the piece of data remains unchanged, will be called systematic or deterministic errors.
Regulations also require that a catastrophic scenario must not result from a single event (a simple failure), whatever the associated probability. Thus it can be seen that, although its probability remains tiny, such a systematic error cannot be tolerated in so far as it may easily result from a simple failure.